"The actual distances are irrelevant," Timothy interrupted. "The problem is one of logic, so assume what you wish."
"Then the sun, being so much more massive, is at the centre. Since the remaining planets, which reflect sunlight, are seen as points they must be much further away than the moon. Now, Mercury and Venus occur both in the morning and evening, but never get far from the sun. That suggests they're closer to the sun than the Earth, and since Mercury always stays closer to the sun, presumably the order out is Mercury, Venus, Earth. To be far enough away, Aristarchus' calculation may be too small. The angle is difficult to measure and. ."
"As I said, for the purposes of debate, if you need the sun to be further away, assume it," Timothy nodded.
"First, the Moon goes around the Earth, so some of the time gets in between the Earth and the Sun, and we get an eclipse. The Moon takes a month to go around the Earth, so every month it has the same phase. This explains the phases. ."
"I concede the Moon goes around the Earth," Timothy interrupted. "My position is, so does everything else. Your arguments on the phases of the Moon will be my arguments."
"Then the planets go around the sun, and simple geometry shows why those closer to the sun can appear either in the morning sky or the evening sky, but not the midnight sky."
"That," Timothy interrupted again, "is probably the strongest point in favour of Aristarchus. However, it has been explained."
"In a very awkward way," Gaius suggested.
"Perhaps," Timothy shrugged.
"Then look at this," Gaius continued. "The outer planets wander across the heavens in a general eastward direction, except very occasionally they seem to stop, then go westwards for a while, then stop and resume going eastwards. This is exactly what Aristarchus predicts, because the reversal occurs when the Earth's motion catches up with the others."
"Nevertheless, that has also been explained," Timothy continued.
"In an even more awkward way," Gaius suggested, "and there's another point. According to Aristarchus the reversal occurs when the planet is at its brightest, and is in the midnight sky. It does, and that requires the epicycles to only occur where the planet has wandered onto a Sun — Earth — planet line. Why do the epicycles only occur there, and where it is almost random against the background?"
"I don't know," Timothy admitted, "but that doesn't make the epicycle model wrong."
"Perhaps, but it certainly doesn't make it right," Gaius emphasized. "Also, there are no planetary eclipses, where the planet falls in the Earth's shadow. Again, the spheres model effectively has to say there are a large number of spheres, but only one is really close. The question is, why is that one special?"
"The same goes for Aristarchus," Timothy protested, too quickly.
"Not at all!" Gaius quickly countered. "Aristarchus says Earth is a planet, and it goes around the sun just like the others, which means that the planets must be a long way away from each other. The Moon falls around the Earth so it has to be much closer. If another planet was, say, two times as far away, the Moon could switch planets, however the planets have no effect on the Moon, nor on our tides. If the other planets are far enough away, the Earth seems to be too small and cannot shade the sun, just as Venus and Mercury are too small to shade the Earth. We know they are too small because we can see them, and see the angle they subtend is far smaller than that of the sun. But for the disc model, there is only one type of disc, the planets have to be on discs a very long way away, and that means there have to be a number of these coincidences with no cause, despite the fact the planets all wander at different speeds."
"You're assuming the planets are very large," Timothy warned. "Just because Aristarchus says so doesn't make it so. They could be small and close."
"In which case they should be eclipsed!" Gaius countered.
"I see," Timothy mused. "They are really good points. In fact, Gaius, you've excelled so far, and if it wasn't for the fact that the Aristarchus model is physically wrong, your arguments on planetary motion would be reasonably convincing."
"You keep saying it's wrong. How?"
"We'll come to that," Timothy assured him. "First, everything goes around the Earth once a day. I assume you will concede that?"
"Everything else more or less stays put," Gaius interposed. "Day/night is explained by the sun being at the centre, and the Earth rotating."
"So let me get this right," Timothy said, a little pedantically, Gaius thought, until he realized that Timothy was overdoing the importance of this. It meant that this would not be his main line of approach. "You are saying that while everything seems to be going around the Earth, it is not, and it is the Earth that is rolling."
"I believe that explains everything quite well," Gaius shrugged. "As Aristotle said, when anything moves, it moves with respect to something else."
"Which is why the Earth has to be immovable," Timothy protested.
"The sun would work just as well," Gaius smiled. "Look at it this way. According to you, all the stars rotate around the Earth at a terrifying velocity?"
"Around an immovable Earth," Timothy confirmed.
"So, supposing something removed the Earth?" Gaius smiled. "What happens? Does everything else just stop? And if so, how does whatever is driving the rotating spheres know when to stop?"
"What?" Timothy gasped. "You can't just remove the Earth."
"You can in the abstract," Gaius said, "and in any case, the problem is merely one of size. The quality is conceptually different from the quantity, as Aristotle noted."
"That may be," Timothy said, as he struggled to recover, "but the fact remains, the Earth is quite different in quality from the rest of the universe."
"You don't know that," Gaius smiled.
"Oh, yes I do!" Timothy responded. "The Earth undergoes continual change, but the other bodies have remained constant for as long as we have been observing them."
"They could be too far away," Gaius replied. "If a body as big as the Earth is reduced to the size of a pin head, you wouldn't see such changes."
"Suppose you see a large storm," Timothy replied. "You can measure the wind speed, and estimate the size of the clouds from the time it take them to pass. If there were clouds that big on the Moon, then we would be able to see them. We would also see them on the edges, but the edges of the Moon are very sharp."
Gaius thought about this for a moment. While this was a point he had never considered, for some reason it should support his position, but he could not for the life of him think how. To gain time he decided to get the conversation back to where it should be, so he said, "We are being distracted. My point is, and I repeat it, that the Moon seems to go slightly slower is because it really does go around, the Earth." He paused, then said in a challenging way, "It explains everything, and there is nothing contradicting it. Therefore, as the great Aristotle would say, it must be true."
"Then let's have some contrary evidence! If the surface of the Earth is moving, there should be a contrary wind," Timothy said firmly, "and if the Earth goes around the sun, there should be a steady wind for that too, different between night and day as the surface is either going in the same direction, or the opposite direction, to its path around the sun."
"Not if the contraries come from the medium the motion is in," Gaius intervened in a triumphant tone.
"Oh?" Timothy was puzzled.
"The motion around the sun is eternal," Gaius smiled, "therefore there is no contrary. If the motion receives its contraries from the medium, and the motion is eternal, then there is no medium. The motion is in a void."